The present invention relates to a process for coating a substrate. More particularly, the invention relates to coating a substrate with a tin oxide-containing material, preferably an electrically conductive tin oxide-containing material and to coated porous substrates.
An application where substrates with coatings, e.g., electrically conductive coatings, find particular usefulness is in the promotion of chemical reactions, e.g., gas/liquid phase reactions, electro catalytic reactions, photo catalytic reactions, redox reactions, etc. As an example of a type of reaction system, a catalytic, e.g., metallic, component is contacted with the material to be reacted, e.g., hydrocarbon, carbon monoxide is passed through or near to the catalytic component to enhance the chemical reaction, e.g., hydrocarbon, carbon monoxide oxidation to carbon dioxide and water and nitrogen oxide reduction to nitrogen. In addition, using a substrate for the catalytic component which is coated with an electrically conductive material is highly advantageous for electro and photoelectron catalysis and/or rapid heat transfer to catalyst surfaces since a field/current can be effectively and efficiently provided to or near the catalytic component for electron transfer reactions. Many types of chemical reactions can be advantageously promoted using such coated substrates. Tin oxide containing coatings on substrates may promote a electron transfer whether or not the chemical reaction is conducted in the presence of a electro photo electro current or field. In addition, tin oxide coated substrates and sintered tin dioxides are useful as gas sensors, and combustion type devices and articles. One or more other components,m e.g., metal components, are often included in certain of these applications.
In many of the above-noted applications it would be advantageous to have an electrically, electronically conductive; electro mechanical tin oxide which is substantially uniform, has high electronic conductivity, and has good chemical properties, e.g., morphology, stability, etc.
A number of techniques may be employed to provide conductive tin oxide coatings on substrates. For example, the chemical vapor deposition (CVD) process may be employed. This process comprises contacting a substrate with a vaporous composition comprising a tin component and a dopant-containing material and contacting the contacted substrate with an oxygen-containing vaporous medium at conditions effective to form the doped tin oxide coating on the substrate. Conventionally, the CVD process occurs simultaneously at high temperatures at very short contact times so that tin oxide is initially deposited on the substrate. However tin oxide can form off the substrate resulting in a low reagent capture rate. The CVD process is well known in the art for coating a single flat surface which is maintained in a fixed position during the above-noted contacting steps. The conventional CVD process is an example of a "line-of-sight" process or a "two dimensional" process in which the tin oxide is formed only on that portion of the substrate directly in the path of the tin source as tin oxide is formed on the substrate. Portions of the substrate, particularly internal surfaces, which are shielded from the tin oxide being formed, e.g., such as pores which extend inwardly from the external surface and substrate layers which are internal at least partially shielded from the depositing tin oxide source by one or more other layers or surfaces closer to the external substrate surface being coated, do not get uniformly coated, if at all, in a "line-of-sight" process. Such shielded substrate portions either are not being contacted by the tin source during line-of-sight processing or are being contacted, if at all, not uniformly by the tin source during line-of-sight processing. A particular problem with "line-of-sight" processes is the need to maintain a fixed distance between the tin source and the substrate. Otherwise, tin dioxide can be deposited or formed off the substrate and lost, with a corresponding loss in process and reagent efficiency.
One of the preferred substrates for use as catalysts including use as a catalyst additive in batteries, such as the positive active material of lead-acid batteries, are inorganic substrates, in particular flakes, spheres, fibers and other type particles. Although the CVD process is useful for coating a single flat surface, for the reasons noted above this process tends to produce non-uniform and/or discontinuous coatings on non-flat, non-equidistant surfaces and/or three dimensional surfaces having inner shielded surfaces and/or the processing is multi-step and/or complex and/or time consuming. Such non uniformities and/or discontinuities and/or processing deficiencies are detrimental to the electrical and chemical properties of the coated substrate. A new process, e.g., a "non-line-of-sight" or "three dimensional" process, useful for coating such substrates would be advantageous. As used herein, a "non-line-of-sight" or "three dimensional" process is a process which coats surfaces of a substrate with tin oxide which surfaces would not be directly exposed to tin oxide-forming compounds being deposited on the external surface of the substrate during the first contacting step and/or to improve the processability to conductive components and articles and/or for the type of substrate to be coated. In other words, a "three dimensional" process coats coatable substrate surfaces which are at least partially shielded by other portions of the substrate which are closer to the external surface of the substrate and/or which are further from the tin oxide forming source during processing, e.g., the internal and/or opposite side surfaces of a glass or ceramic fiber or spheres, or flakes or other shapes or surfaces.
Although a substantial amount of work has been done, there continues to be a need for a new method for coating substrates, particularly three dimensional substrates with tin oxides. The prior art processes described below follow conventional processing techniques such as by sintering of a tin oxide and/or the instantaneous conversion to tin oxide by spray pyrolysis.
For example in "Preparation of Thick Crystalline Films of Tin Oxide and Porous Glass Partially Filled with Tin Oxide," R. G. Bartholomew et al, J. Electrochem, Soc. Vol. 116, No. 9, p1205(1969), a method is described for producing films of SnO.sub.2 on a 96% silica glass substrate by oxidation of stannous chloride. The plates of glass are pretreated to remove moisture, and the entire coating process appears to have been done under anhydrous conditions. Specific electrical resistivity values for SnO.sub.2 -porous glass were surprisingly high. In addition, doping with SbCl.sub.3 was attempted, but substantially no improvement, i.e., reduction, in electrical resistivity was observed. Apparently, no effective amount of antimony was incorporated. No other dopant materials were disclosed.
In "Physical Properties of Tin Oxide Films Deposited by Oxidation of SnCl.sub.2," by N. Srinivasa Murty et al, Thin Solid Films, 92(1982) 347-354, a method for depositing SnO.sub.2 films was disclosed which involved contacting a substrate with a combined vapor of SnCl.sub.2 and oxygen. Although no dopants were used, dopant elements such as antimony and fluorine were postulated as being useful to reduce the electrical resistivity of the SnO.sub.2 films.
This last described method is somewhat similar to the conventional spray pyrolysis technique for coating substrates. In the spray pyrolysis approach tin chloride dissolved in water at low pH is sprayed onto a hot, i.e., on the order of about 600.degree. C., surface in the presence of an oxidizing vapor, e.g., air. The tin chloride is immediately converted, e.g., by hydrolysis and/or oxidation, to SnO.sub.2, which forms a film on the surface. In order to get a sufficient SnO.sub.2 coating on a glass fiber substrate to allow the coated substrate to be useful as a component of a lead-acid battery, on the order of about 20 spraying passes on each side have been required. In other words, it is frequently difficult, if not impossible, with spray pyrolysis to achieve the requisite thickness and uniformity of the tin oxide coating on substrates, in particular three dimensional substrates.
Dislich, et al U.S. Pat. No. 4,229,491 discloses a process for producing cadmium stannate layers on a glass substrate. The process involved dipping the substrate into an alcoholic solution of a reaction product containing cadmium and tin; withdrawing the substrate form the solution in a humid atmosphere; and gradually heating the coated substrate to 650.degree. C. whereby hydrolysis and pyrolysis remove residues from the coated substrate. Dislich, et al is not concerned with coating substrates for lead-acid batteries, let alone the stability required, and is not concerned with maintaining a suitable concentration of a volatile dopant, such as fluoride, in the coating composition during production of the coated substrate.
Pytlewski U.S. Pat. No. 4,229,491 discloses changing the surface characteristics of a substrate surface, e.g., glass pane, by coating the surface with a tin-containing polymer. These polymers, which may contain a second metal such as iron, cobalt, nickel, bismuth, lead, titanium, canadium, chromium, copper, molybdenum, antimony and tungsten, are prepared in the form of a colloidal dispersion of the polymer in water. Pytlewski discloses that such polymers, when coated on glass surfaces, retard soiling. Pytlewski is not concerned with the electrical properties of the polymers or of the coated substrate surfaces.
Gonzalez-Oliver, C. J. R. and Kato, I. in "Sn (Sb)-Oxide Sol-Gel Coatings of Glass," Journal of Non-Crystalline Solids 82(1986) 400-410 North Holland, Amsterdam, describe a process for applying an electrically conductive coating to glass substrates with solutions containing tin and antimony. This coating is applied by repeatedly dipping the substrate into the solution of repeatedly spraying the solution onto the substrate. After each dipping or spraying, the coated substrate is subjected to elevated temperatures on the order to 550.degree. C.-600.degree. C. to fully condense the most recently applied layer. Other workers, eg., R. Pryane and I. Kato, have disclosed coating glass substrates, such as electrodes, with doped tin oxide materials. The glass substrate is dipped into solution containing organo-metallic compounds of tin and antimony. Although multiple dippings are disclosed, after each dipping the coated substrate is treated at temperatures between 500.degree. C. and 630.degree. C. to finish off the polycondensation reactions, particularly to remove deleterious carbon, as well as to increase the hardness and density of the coating.